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Information on Drying of a Concrete Slab

  • Posted by: Alex Wilkie

Best conditions for drying of concrete slabs

There are four main factors which influence the rate at which a concrete slab dries. These are: time, temperature of material, airflow over material and difference in relative humidity between the slab and the air.  All four of these factors should be controlled in order to effectively dry a concrete slab.

The ideal conditions for drying of a concrete slab are:

  • high airflow across the entire surface of the slab
  • The temperature of the slab raised several degrees (to approximately 40°C)
  • Relative humidity of the air surrounding the slab as low as possible
  • A reasonable amount of time allowed for the slab to dry

Fastest drying option

A desiccant dehumidifier is the most effective piece of equipment for drying of a concrete slab, and can lower the relative humidity to near 0%. If a desiccant dehumidifier is not an option other dehumidifiers can be used, however they are less effective. For example, Low Grain Refrigerant (LGR) type dehumidifiers can lower relative humidity to around 25%.  Additionally, Conventional dehumidifiers are even less effective and can only lower relative humidity to around 35%. For relative humidity, the lower the percentage (i.e. 0%) the lower the moisture content in the air. In drying of a concrete slab, lower relative humidity means the slab will dry faster.

Desiccant dehumidifiers combined with air movement would be the most effective way to quickly dry a concrete slab. However, caution should be used as over-drying of the slab is a possibility.  As a result, the process should be closely monitored by experienced technicians. Minimising the volume of air (and concrete) being dried is a practical way to make drying more effective and efficient. Plastic sheeting is normally used to create a thin layer of dry air above the slab. Doing this helps reduce the volume of air being dried. Furthermore, it also helps the equipment cope with the size of the environment.

Most cost effective drying option

Waiting for the slab to cure by natural means would be the most cost-effective method of drying the slab. However, time constraints are often a factor – especially on building sites.  Using LGR dehumidifiers combined with air movers in a closed environment can have some success in drying the slab, however it could still take several weeks for the slab to dry.  Alternatively, heating the slab in an open environment using infrared lamps may also help, however this method relies heavily on good ambient conditions.

Australian standards for flooring installation

The Australian Standards for relative humidity (AS 1884-2012) in a concrete slab before installation of flooring states that relative humidity should not exceed 75% (Section A3.1.2 “Relative humidity in-situ probe test”).  The levels of relative humidity in concrete slabs should be monitored according to ASTM F2170-18 (also referenced in AS 1884-2012 Section A3.1.1 “Test methods”). Failure to follow these recommendations may lead to issues post installation.  These may include damage to resilient flooring, bubbling of flooring, flooring lifting or even mould and microbial growth on the underside of flooring.

IECL can provide assessment and testing of concrete moisture according to Australian standards. IECL can also provide recommendations of drying strategies or create a scope of works in how to dry the concrete slab.

References

  1. AS 1884-2012, “Floor Coverings – Resilient sheet and tiles – Installation Practices”
  2. ASTM F2170-18, “Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using In Situ Probes”